Tunable matching network

ABSTRACT

A matching network is to be provided which can quickly and easily be tuned to a desired impedance. The matching network has a first and a second line which are interconnected at one end, while their other ends are coupled to a microwave line, and a third line which branches off from the interconnection of the other two lines. The first and/or second line and the third line are loaded with ferrite. The ferrite of the first and/or second line and that of the third line are exposed to separate magnetic fields which can be varied independently of each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tunable matching network which may becoupled to a microwave transmission line.

2. Brief Description of the Related Art

As indicated in a presentation by F. Durodie on New Antenna ImpedanceEvaluation and Matching Tools for TEXTORS's ICRH System, at the 16thSymposium on Fusion Technology (SOFT), London, Sep. 3-7, 1990, a tunablematching network, is required, for example, for a microwave transmissionline which couples high power microwave energy into the plasmacombustion chamber of a fusion reactor. Since the plasma combustionchamber represents a constantly changing load resistance to themicrowave transmission line and in order for the generator generatingthe microwave energy not to be damaged by reflections which are theresult of a mismatch, each occurring load resistance must be transformedto the characteristic impedance of the line. According to the mentionedpublication, two tunable capacitors which are separated from one anotherby the length of a transformation line, which must be measuredprecisely, are coupled to the microwave transmission line for thispurpose. Tuning of the capacitors is the result of a mechanicallyelaborate pneumatic device. However, since the load resistance maychange very rapidly, this arrangement would be too slow to bring aboutmatching that is as free of delay as possible.

A tunable matching network may not only be used in the case described,but at any time a changing resistance impedance is switched on to amicrowave transmission line.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a matching network whichmay be rapidly tuned to a desired impedance at low expense.

According to the invention, this object is attained by a tunablematching network having first and second lines each having a first and asecond end, the first ends of the first and second lines being connectedtogether and the second ends of the first and second lines each beingadapted for coupling to a microwave transmission line, and at least oneof the first and second lines being loaded with ferrite material, athird line having one end coupled to and branching off from the firstends of the first and second lines and being loaded with the ferritematerial, and means for generating and exposing the ferrite material ofthe first and second lines and the ferrite material of the third line toseparate magnetic fields which are independently chargeable for turningthe matching network.

Due to the fact that the matching network may be tuned electricallywithout any mechanically movable parts, impedance matching that is freeof delay is ensured when the load resistance of the microwavetransmission line changes rapidly.

A further advantage of the arrangement is that no transformation line isrequired between the two variable reactances of the matching networkmentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further elucidated by means of an embodiment shown inthe drawing in which:

FIG. 1 is a longitudinal view of a matching network and

FIG. 2 is a perspective illustration of the same,

FIG. 3 is an equivalent circuit diagram of this matching network.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a longitudinal section and FIG. 2 is a perspectiveillustration of a tunable matching network, which is coupled to amicrowave transmission line L. In the illustrated embodiment, themicrowave transmission line L is a coaxial line having an innerconductor LI. As already mentioned in the introduction, and asillustrated by the equivalent circuit diagram in FIG. 3, the microwavetransmission line L is fed at its input by a generator G and isterminated at its opposite output by means of a changing load resistanceZL. The T-equivalent circuit diagram which includes impedances Z1 andZ2, and which is inserted into the microwave transmission line L,represents the matching network, which serves to transform therespective load resistance ZL to the characteristic impedance of theline.

The matching network has a first line L1 and a second line L2, each ofwhich contacts with one end of the interrupted inner conductor LI of thecoaxial microwave transmission line L. At the opposite end, the twolines L1 and L2 are connected to one another. A third line L3 branchesoff from this connecting point. In the embodiment shown in FIGS. 1 and2, lines L1, L2 and L3 are configured as strip conductors. The outerconductor to the strip conductors L1, L2 and L3 is formed by the housingGS, which is indicated by hatching and which is connected to the outerconductor of the coaxial microwave transmission line L. In the shownembodiment, the plate-shaped inner conductors of the two strip lines L1and L2 are coated with ferrite layers F1 and F2 on adjacent faces. Inthe third line L3, the plate-shaped inner conductor is coated on bothsides with ferrite layers F31 and F32. Instead of applying ferritelayers F1, F2, F31, F32 to the inner conductors, the outer conductor GSof the three lines may be also coated with ferrite. The same appliesalso if lines L1, L2 and L3 are realized as coaxial lines. The arrowsdrawn in FIG. 1 outside the matching network indicate that the two linesL1 and L2 are exposed to a magnetic field M1, and separated from this,the third line L3 is exposed to a magnetic field M2. What is involvedare magnetic fields M1 and M2 which can be changed independently of oneanother. With the magnetic field M1 acting on lines L1 and L2, theelectrical length of these two lines L1 and L2 may be varied.Independently of this, the electrical length of the third line L3 may bevaried by means of the changeable magnetic field M2 which influencesferrites F31 and F32.

The described arrangement of lines L1, L2 and L3 actually represents twodifferent networks. The one network comprising the first line L1 andsecond line L2, together with the housing GS, forms a shielded two-wireline in which two modes exist, an in-phase mode and a push-pull mode.The push-pull mode is present if the currents flowing in lines L1 and L2are equally strong and flow in opposite directions, and the in-phasemode is present if the currents flowing in lines L1 and L2 are equallystrong and directed in the same direction.

In the second network, comprising line L3 and the housing GS, only thein-phase mode is able to propagate. The ferrite material on lines L1 andL2 is arranged between the lines (see FIG. 1) and thus is only effectivefor the push-pull mode. The push-pull impedance Zg of lines L1, L2 istuned by means of magnetic field M1, and the in-phase impedance Zs ofline L3 by means of magnetic field M2.

The impedances Z1 and Z2 indicated in the equivalent circuit diagram(see FIG. 3) of the matching network then have the followingrelationship to the in-phase impedance Z_(s) and to the push-pullimpedance Z_(g) : ##EQU1##

In case the matching network is operated at very high power, it isadvisable to cool lines L1, L2 and L3. The heat generated in ferritesF1, F2, F31 and F32 can be dissipated very effectively and in a simplemanner with the help of cooling channels that pass through the innerconductor and/or the outer conductor of lines L1, L2 and L3 which areconfigured as strip lines or coaxial lines. FIG. 1 indicates a coolingchannel designated K.

The changeable magnetic fields M1 and M2 are produced by controllableelectromagnets. However, additional permanent magnets may also beprovided which produce a static magnetic field of such strength that theferrites are operated above their gyromagnetic resonance where they showthe least losses. The use of permanent magnets and electromagnets hasthe advantage that for tuning the ferrite loaded lines only smallcurrents are required because, thanks to the permanent magnets, only aportion of the required magnetization must be generated by theelectromagnets. It is also advantageous that, during a possible failureof the control current for the electromagnets, the leakage power in theferrites does not rise very much, because the permanent magnets alwaysmaintain the magnetization of the ferrites above the gyromagneticresonance.

We claim:
 1. A tunable matching network for coupling to a microwavetransmission line, comprising:first and second transmission lines eachhaving a first end and a second end, the first ends of the first andsecond transmission lines being connected together and the second endsof the first and second transmission lines each being adapted forcoupling to a microwave transmission line, and at least one of the firstand second transmission lines being loaded with ferrite material; athird transmission line having one end coupled to and branching off fromthe first ends of the first and second transmission lines and beingloaded with ferrite material; and means for generating and exposing theferrite material of the first and second transmission lines and theferrite material of the third transmission line to separate magneticfields which are independently changeable for tuning the matchingnetwork.
 2. The tunable matching network according to claim 1, whereinthe first, second and third transmission lines are coaxial conductorseach having an inner conductor and an outer conductor, and at least oneof the inner conductor and the outer conductor of each of the first,second and third transmission lines are at least partially coated withthe ferrite material.
 3. The tunable matching network according to claim1, wherein the first, second and third transmission lines are striplines each having an inner conductor and an outer conductor, and atleast one of the inner conductor and outer conductor of each of thefirst, second and third transmission lines are at least partially coatedwith the ferrite material.
 4. The tunable matching network according toclaim 1, wherein the first, second and third transmission lines eachinclude an inner conductor and an outer conductor, the tunable matchingnetwork further comprising a cooling channel passing through at leastone of the inner conductor and the outer conductor of each of the first,second and third transmission lines.
 5. The tunable matching networkaccording to claim 1, wherein the ferrite material is loaded on both thefirst and second transmission lines.
 6. The tunable matching networkaccording to claim 1, wherein at least one of the first and secondmagnetic fields include a permanent magnetic field component and avariable magnetic field component.
 7. A tunable matching network forcoupling to a microwave transmission line, comprising:first and secondtransmission lines each having a first end and a second end, the firstends of the first and second transmission lines being connected togetherand the second ends of the first and second transmission lines eachbeing adapted for coupling to a microwave transmission line, and atleast one of the first and second transmission lines being loaded withferrite material; a third transmission line having one end coupled toand branching off from the first ends of the first and secondtransmission lines and being loaded with ferrite material; and means forgenerating and commonly exposing the ferrite material of the first andsecond transmission lines to a first magnetic field and for generatingand exposing the ferrite material of the third transmission line to asecond magnetic field, the first and second magnetic fields beingseparate and independently changeable for tuning the matching network.